I'm about to spend a significant (for my finances) amount of money on a new Canon full frame body, it is going to be either a 6D or a 5D3. The plan is to get the camera body and 2 very fast primes: Sigma 35/1.4 and Canon 85/1.2. The 6D has got what i want and costs about 1000€ less than the 5D3, but given the fact that the DoF with fast primes is going to be tiny to say the least, how is the supposedly "simplistic" 6D AF system going to perform? The main fields of application will be travel and street photography, family portraits, some landscapes, the occasional event, and everything will happen just for the sake of it, i'm not trying to make a living out of my hobby (or else i wouldn't have such quantity of money to squander on gear ). I just don't want to spend a significant amount of money and be disappointed by a quirky AF behavior. I would like to hear from owners of these cameras and lenses, lots of online researches and reading still aren't enough to make up my mind.

It depends on your technique for using autofocus. Generally just leaving any camera to do all point full auto is no good as it will sometimes choose the wrong thing to focus on. So a lot of people just use the center point to pull focus then recompose if needed, this would work fine with 6d. However, if you are the type that likes to cycle between af points and use the outer ones frequently the 5dmkiii is much better for this. Or if you use zone/servo af for moving targets 5diii will be a lot better for this.

Since you mentioned you are into super shallow DOF photography, relying on an AF system regardless of how advanced it is, isn't going to be all you need. Manual Focus isn't a dirty word either. The 6D allows you to change focusing screens so you can preview the true DOF at really wide apertures. Which has value to some. I enjoy using a 5DII and 85 1.2II combo with Eg-S focusing screen and back button AF set on my camera, then I just use my left hand to constantly tweak focus (and since the shutter button doesn't control AF now, it's not competing with me!)

I would not use servo as it's slow with the Sigma lens. With slow subjects and one shot focus, you get the photos you want. If you take pics of fast moving kids or something similar, you might get better results with the 35L.

The 85 1.2 L is very slow at focusing and the version II is also quite slow.

I love the 35mm focal length, although I bought my 35mm f1.4 L before the new Sigma came out so I don't have any hands-on experience with the Sigma.

I'm surprised by your choice of the 85mm f1.2L. Many pros have a love-hate relationship with the lens. I've heard more than one say something on the lines of: "It's expensive, heavy and slow to focus."

Have you already considered the 135mm f2L. The bokeh is beautiful (similar to 85mm) and it's generally a much easier lens to live with. It's lighter, faster-focusing and half the price of the 85mm. Another option to consider is Simga's 85mm f1.4, which has good reviews. I have gone through a similar decision-making process a while back and opted for 135mmL +600EX-RT flash and softbox-setup.

Most of the type of photography you mention doesn't really require wide apertures. Landscapes, for example tend to just look out of focus with super wide apertures.

If I was doing "street" photography, I'd want a light, easy to use unobtrusive camera. I used to own an EF 50mm f1.0 and an EF 85mm f1.2 L, the original design. I did theatrical photography at the time. You pre-focus, and the final focus is quite fast with either lens when you are within a few inches of correct focus as you will be when you pre-focus (by holding the shutter button down 1/2 way).

At the time I was using film, and today I have a couple T3i bodies, but I'd probably consider one of the nearly full frame non-reflex cameras for the type of use you describe. I'd want something I'd always have with me. Today this is my cellphone.

A bit of an exaggeration... I often use the outer focal points on my 6D. Yes, the center point is much better, but the outer points are not bad in decent light. I can nail shallow DOF with the outer points.

Thank you all for the feedback. I see most of the 6D users here use the center focus point and recompose, which is something i wouldn't like to do. MF could be a better option. Is there any advantage using the viewfinder with a dedicated focusing screen over the live view?

The difference is in how you can use the camera. Just imagine standing with the camera somewhat in front of you for using live view. If you want to do shallow depth of field photography you better put the camera on a tripod or use some other form of support. Using a good dedicated focusing screen you can hold the camera up to your eye and focus, holding the camera steady without requiring extra support.

Although I have been using manual focus for more than two decades, so I guess I am pretty much used to it, I would still prefer using a 5D Mark III to using a 6D with a dedicated focusing screen. The difference is being able to quickly select a focusing point and then to use it effectively to focus the lens without further ado on the 5d Mark III. This is not only much more convenient, but will also allow you to avoid errors.

A bit of an exaggeration... I often use the outer focal points on my 6D. Yes, the center point is much better, but the outer points are not bad in decent light. I can nail shallow DOF with the outer points.

Exactly - "can", but not reliably so. I also use the outer points for static subjects to avoid focus shift when recomposing, but I want to be more sure that I end up in focus I use center only. On anything that moves the outer points are near useless though since they are not cross, at least in my experience with the 6d.

MF could be a better option. Is there any advantage using the viewfinder with a dedicated focusing screen over the live view?

No, not if you're using Magic Lantern which has "focus peaking" in live view and shows you _exactly_ what's in focus, I use it all the time for macro, couldn't do without it.

As for the Eg-S screen (though: I'm using the Eg-D with a grid to improve my verticals :-p): It's near-essential for mf with fast lenses, but as far as I've researched it don't expect wonders from it either and it does darken the screen for slower lenses - so you probably have to try for yourself. The option to use Canon screens (and not expensive 3rd party) and quickly exchange them is one of the few advantages of the 6d over the 5d3 though.

Neuro has pointed out a problem of using the centre point and then recomposing. It stuck in my mind, and here is a simple explanation. A lens is designed so that everything in a plane perpendicular to the lens is in focus. So, focus on the centre of a brick wall in front of you (flat facing you, of course), and all the bricks are in focus, including the ones at the edges, which are actually farther away. Focus on a brick at the edge, hold the focus and point to the middle, then the focus will actually be behind the wall because the bricks at the end were farther away when you focussed. So, if you focus and recompose, the subject might not be in good focus even if it is in the same plane.

Regarding 1Dx and 5DIII, I thought that they had the same AF system, which was probably the biggest plus of the 5DIII.